If you connect the body to a spring, which is neither compressed nor extended, there's no force acting on it.

如果将一个弹簧连在这个物体上,弹簧既没有被压缩也没有被拉伸,也不会有力作用在它上面

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For me, I like to walk here, I like to run, I do stretches,

对我来说，我喜欢在这儿走路，跑步，拉伸。

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Newton is not going to come and tell you what force the spring will exert when it's pulled by some amount.

牛顿不会过来告诉你,弹簧被拉伸一定量时会产生多大的力

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I draw a graph here of the amount by which I pull the spring versus the force it exerts.

我在这里画个图,纵轴是弹簧的拉伸量,横轴是力的大小

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Now I can measure that, because I pull it by one centimeter and I find the acceleration it exerts on a known mass.

现在就能测量这个力了,因为把弹簧拉伸 1 厘米,我就知道它让一个质量已知的物体产生的加速度

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We want to know how much force it exerts when I pull it by a certain amount.

我们想知道当我们将弹簧拉伸一定量时,它能产生多大的力

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If it's positive, it means you've stretched it, if it's negative, it means compressed.

如果它为正值,就说明弹簧被拉伸了,如果它为负值,就说明弹簧被压缩了

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Namely, if you pull it by so much, that's the force it will exert.

也就是,将弹簧拉伸这么长,它产生的力就有这么大

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You hook one end of the spring to a wall and you pull it from rest by some amount and you attach the one kilogram mass to it.

你把弹簧的一端固定在墙上,然后拉伸一段距离,接着你把 1 千克物体固定在上面

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Then I pull it by 1.1 cm, and I find ma.

然后我把弹簧拉伸 1.1 厘米,得到 ma

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When you say the rope is under tension being pulled from both sides by a certain force, the tension is not 0 just because this T and that T cancel.

当绳子被绷紧,即两端被拉伸时,拉力并不为 0,因为这个 T 和那个 T 相互抵消了

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I find 1.2, I find ma.

拉伸1.2厘米,得到 ma

耶鲁公开课 - 基础物理课程节选